I was and still am tempted to pass this one up. Seems to be something you know about the car that is not mentioned. It may be in your subconcious with lots of experience. Or it may not exist at all. At a twenty five percent or more decline from normal average fuel milage there should be a clue. Have you really made sure car is not dumping fuel at speed. Might not hurt to bring engine up to mid speed in your driveway for a minute and have a careful look for any fuel drips on the ground. It might not leak at idle. Disable the egr valve and make sure it is closed up as well. Probably done already. Number one element on any older pump is the most likely to be off if any element is. The result of too much aggression or not using common sense for timing attempts in the distant past for example. Does car feel normally peppy compared to other examples you have driven? Idle better, average or worse than other 617 engines you are familar with? No I do not disagree with your thought it might be the injectors or pump either. Or an amalgamation of many sub standard things. As for the injectors that have been out in the weather for a couple of months I just do not know. Oil would have been still in them and any water introduced by rainfall may have kind of homoginized with the existing oil. They might not be bad. If you can get them at a favorable price (nothing or near to it) why not give them a try. Nothing to loose but a little time and I can see no harm really. I suspect the approach you want to take is to narrow it down or find the problem by elimination. How does this engine start at cooler temperatures? This might provide the biggest clue or eliminate some things as well. For example an engine with severly retarded timing will be much harder starting I believe compared with cars of almost equal compression. In cases where I just do not know or have any pretty obvious prime suspects. Ocasionally a good tuneup sometimes reveals the problem or reduces the possibilities to more managable proportions. Make sure your return line to the tank is not obstructed as well.